El'gygytgyn, a Russian lake 100 kilometers north of the Arctic Circle, contains layers of sediment that date back to the lake's formation 3.6 million years ago. Analyses of sediment cores have revealed that back then summers reached about 15 to 16 degrees Celsius, a good 8 degrees warmer than modern Arctic summers. These warm temperatures, which supported plants like Douglas fir and hemlock, lasted until about 2.2 million years ago.

Using a sediment core as a detailed history of climate change, scientists can see how the forested Arctic gradually became covered in ice and snow. These changes help us understand details about the development of Ice Ages. In addition, the sediment comes from a window of time during the Pliocene Epoch, when greenhouse gas levels were only slightly higher than they are today. Such sensitivity to small carbon dioxide changes hint at a warm Arctic future.